1. Field of the Invention
The present invention relates to an image sensor, an imaging system, and an image sensor control method.
2. Description of the Related Art
Imaging systems such as a digital cameras and video camcorders sometimes adopt a MOS image sensor. In some cases, a MOS image sensor S will have an arrangement as shown in FIG. 10.
The image sensor S includes a pixel array PA, a vertical scanning circuit 15, noise reduction circuits 10-1 to 10-m, horizontal transfer switches 16-1 to 16-m, a horizontal scanning circuit 14, and an output amplifier 17.
In the pixel array PA, a plurality of pixels P11 to Pnm are arrayed two-dimensionally (in the row and column directions).
The vertical scanning circuit 15 scans the pixel array PA in the vertical direction via row control lines. The row control lines include selection row control lines 101-1 to 101-n, reset row control lines 102-1 to 102-n, and transfer row control lines 103-1 to 103-n. The vertical scanning circuit 15 selects a row of the pixel array PA.
The noise reduction circuits 10-1 to 10-n read out N and S signals at different timings via column signal lines 8-1 to 8-n from the pixels of respective columns on a selected row. The noise reduction circuits 10-1 to 10-n execute CDS processing to generate the difference signals between N and S signals, obtaining noise-free signals.
The horizontal scanning circuit 14 sequentially turns on the horizontal transfer switches 16-1 to 16-n to sequentially transfer the difference signals of the pixels of respective columns on a selected row to the output amplifier 17.
The output amplifier 17 amplifies the difference signals, and outputs them from an output terminal 18.
The arrangement of each pixel P11 to Pnm in the pixel array PA will be explained with reference to FIG. 11. Although the arrangement of the pixel P11 will be exemplified, the arrangements of the remaining pixels P12 to Pnm are also equivalent to that of the pixel P11.
The pixel P11 includes a photodiode 1, amplification MOS transistor 2, selection MOS transistor 3, reset MOS transistor 4, transfer MOS transistor 6, and floating diffusion (FD) 7.
The photodiode 1 generates and accumulates charges corresponding to incident light. Upon receiving an active-level transfer signal from the vertical scanning circuit 15 via the transfer row control line 103-1, the transfer MOS transistor 6 is turned on to transfer the charges of the photodiode 1 to the FD 7. The FD 7 converts the transferred charges into a voltage in accordance with its own parasitic capacitance. Upon receiving an active-level selection signal from the vertical scanning circuit 15 via the selection row control line 101-1, the selection MOS transistor 3 is turned on to select the pixel P11. Upon receiving an inactive-level selection signal from the vertical scanning circuit 15 via the selection row control line 101-1, the selection MOS transistor 3 is turned off to deselect the pixel P11. In the selected state, the amplification MOS transistor 2 performs a source follower operation together with a constant current source 5 connected to the column signal line 8-1, outputting a signal corresponding to the voltage of the FD 7 to the column signal line 8-1. Upon receiving an active-level reset signal via the reset row control line 102-1, the reset MOS transistor 4 is turned on to reset the FD 7. The drains of the reset MOS transistor 4 and amplification MOS transistor 2 are connected to a power supply line SL for supplying constant power.
Recently, there has been a need to increase the number of pixels in pixel array of an image sensor. To increase the number of pixels in a predetermined area which is required for the pixel array, the pixel size is sometimes decreased. In each pixel P11 to Pnm, if the gate width and gate length of the amplification MOS transistor 2 are decreased along with this, random noise in a signal output from the amplification MOS transistor 2 in each pixel P11 to Pnm to each column signal line 8-1 to 8-n specifically increases as shown in FIG. 12.
FIG. 12 shows the distribution of random noise in the pixel array PA that is obtained by processing the dark outputs (N signals) of a plurality of frames. In FIG. 12, the abscissa axis represents the magnitude (standard deviation of dark outputs) of random noise of each pixel, and the ordinate axis represents the number of pixels.
FIG. 12 shows that, in region A, the first component, which exhibits almost a normal distribution, is contained in the random noise. The first component fluctuates randomly between pixels in each frame.
FIG. 12 shows that, in region B, the second component, which exhibits an exponential distribution, is contained in the random noise. The second component will be called flicker noise. As shown in FIG. 13, a specific pixel generates flicker noise (second component) with a specific magnitude (much larger than the first component).
FIG. 13 shows temporal fluctuations of a dark output from a specific pixel in region B. As shown in FIG. 13, flicker noise tends to repeat two, H-and L-level values at random.
Flicker noise is associated with 1/f noise depending on the driving frequency, and often generated from a MOS transistor in a pixel. More specifically, flicker noise depends on the product (gate area) of the gate length L and gate width W of the MOS transistor. In the amplification MOS transistor 2 in which the gate area is set small, flicker noise of a specific pixel increases specifically in the plurality of pixels. Accordingly, random noise including the flicker noise in a signal output from the amplification MOS transistor 2 of a specific pixel to the column signal line sometimes increases specifically in the plurality of pixels.
Flicker noise of a specific pixel stands out as a white spot in an image obtained upon low-illuminance shooting. In particular, a white spot becomes very conspicuous in an image obtained by movie video shooting at low illuminance.
To solve this problem, a reference “M. F. Snoeij, et al., “The Effect of Switched Biasing on 1/f Noise in CMOS Imager Front-Ends”, (US), 2005 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors, June 2005, pp. 68-71” (to be referred to as non-patent reference 1 hereinafter) proposes switching an amplification MOS transistor in the pixel of an image sensor between two, ON and OFF states by a switched basing method. More specifically, before the amplification MOS transistor of a pixel outputs a signal to a column signal line, a switching circuit connected to the column signal line turns off the amplification MOS transistor by increasing the electric potential of the column signal line to VDD once. According to the non-patent reference 1, 1/f noise of the amplification MOS transistor can be reduced, reducing flicker noise.
Japanese Patent Laid-Open No. 2005-322901 proposes a method of controlling the substrate electric potential of an amplification MOS transistor in an image sensor to change the amplification MOS transistor in a pixel to an accumulation mode and turn it off, and then change the amplification MOS transistor to a strong inversion mode and turn it on. According to Japanese Patent Laid-Open No. 2005-322901, before turning on the amplification MOS transistor, trapping of charges in the substrate surface can be inactivated, reducing flicker noise.
Japanese Patent Laid-Open No. 2007-060500 proposes a method of changing the electric potential of an FD to the ground electric potential before turning on an amplification MOS transistor in an image sensor. According to Japanese Patent Laid-Open No. 2007-060500, before turning on the amplification MOS transistor, it can reliably change to an accumulation state, reducing flicker noise.
The technique described in the non-patent reference 1 may not be able to reduce flicker noise when the number of pixels in the pixel array increases. More specifically, as the number of selection MOS transistors connected to each signal line increases, their parasitic capacitances become non-negligible. As a result, it becomes difficult to quickly increase the electric potential of the column signal line to the electric potential VDD or quickly decrease it to a predetermined electric potential by a switching circuit connected to a column signal line. This provides a possibility that no amplification MOS transistor can be turned off/on quickly and that thereby no flicker noise may be reduced.
Also, the technique disclosed in Japanese Patent Laid-Open No. 2005-322901 may not be able to reduce flicker noise if the number of pixels in the pixel array increases. More specifically, as the number of amplification MOS transistors in the pixel array increases, their substrate capacitances become non-negligible. It becomes difficult to increase the substrate electric potential of the amplification MOS transistor from low level to high level or decrease it from high level to low level quickly. This provides a possibility that no amplification MOS transistor can be turned off/on quickly and that thereby no flicker noise may be reduced.
The technique disclosed in Japanese Patent Laid-Open No. 2007-060500 turns off/on an amplification MOS transistor by controlling the electric potential of an FD to the ground electric potential/power supply electric potential. This provides a possibility that a signal containing flicker noise generated after changing the amplification MOS transistor from the OFF state to the ON state is sometimes output to a signal line and that thereby no flicker noise may be reduced.
For example, when reading out an S signal from a pixel, the amplification MOS transistor changes from the OFF state to the ON state by controlling the electric potential of the FD from the ground electric potential to the power supply electric potential. Then, the transfer MOS transistor is turned on to transfer the charges of the photodiode to the FD. The amplification MOS transistor outputs a signal (S signal) corresponding to the voltage of the FD to the column signal line. That is, after the amplification MOS transistor is turned on to transfer the charges of the photodiode to the FD, it outputs a signal corresponding to the voltage of the FD to the signal line. This provides a possibility that the signal output to the signal line highly likely contains flicker noise.